Grafting of unsaturated monomer molecules onto olefin polymers and copolymers has been disclosed in a number of patents. The grafting technique has been used to impart changes in the polymer to which the grafted molecules are attached.
With respect to the invention described and claimed in this application, it is believed :hat the following patents are representative of the most relevant prior art on grafting of which we are aware: U.S. Pat. Nos. 2,970,129; 3,177,269; 3,270,090; 3,873,643; 3,882,194; 3,886,227; 4,087,587; 4,087,588; 4,239,830; 4,298,712; 4,394,485; 4,762,890; U.K. 2,081,723; Jap. Kokai 49(1973)-129742.
The principal distinctions between low density polyethylene (LDPE), high density polyethylene (HDPE), and linear low density polyethylene copolymer (LLDPE) are well-known to practitioners of the polyethylene art and are described, e.g., in U.S. Pat. No. 4,327,009.
There are, basically, two types of olefin polymerization techniques for preparing high molecular weight olefin polymers and copolymers. The oldest commercial technique involves high pressure, high temperature, and the use of a free radical initiator, such as a peroxide; these type polymers are generally known as low density polyethylene (LDPE) and are also known as ICI-type polyethylenes. These LDPE polymers contain branched chains of polymerized monomer units pendant from the main polymer "backbone" and generally have densities in the range of about 0.910-0.935 grams/cubic centimeter (gms/cc).
The other commercially-used technique involves coordination catalysts of the "Ziegler" type or "Phillips" type and includes variations of the Ziegler type, such as the Natta type. These catalysts may be used at very high pressures, but may also (and generally are) used at very low or intermediate pressures. The products made by these coordination catalysts are generally known as "linear" polymers because of the substantial absence of branched chains of polymerized monomer units pendant from the main polymer "backbone" and they are also generally known as high density polyethylene (HDPE). It is these "linear" polymers to which the present invention pertains. Linear high density polyethylene (HDPE) ordinarily has a density in the range of 0.941 to 0.965 gms/cc.
In some of the blends of the present invention there is used a "linear" type ethylene polymer wherein ethylene has been polymerized along with minor amounts of alpha, beta-ethylenically unsaturated alkenes having from 3 to 12 carbons per alkene molecule, preferably 4 to 8 and most preferably 8 carbons per alkene molecule (i.e., 1-octene). The amount of the alkene comonomer is generally sufficient to cause the density of the polymer to be substantially in the same density range as LDPE, due to the alkyl side chains on the polymer molecule, yet the polymer remains in the "linear" classification; they are conveniently referred to as "linear low density polyethylene" (LLDPE). These polymers retain much of the strength, crystallinity, and toughness normally found in HDPE homopolymers of ethylene, but the higher alkene comonomers impart high "cling" and "block" characteristics to extrusion-cast films and the high "slip" characteristic inherently found in HDPE is diminished.
The use of coordination-type catalysts for polymerizing ethylene into homopolymers or copolymerizing ethylene with higher alkenes to make copolymers having densities above about 0.94 gms/cc (i.e., "HDPE" polymers) and/or for copolymerizing ethylene with higher alkenes to make copolymers having densities in the range of LDPE and medium density polyethylene (i.e., "LLDPE" copolymers) is disclosed variously in, e.g., U.S. Pat. Nos. 2,699,457; 2,862,917; 2,905,645; 2,846,425; 3,058,963 and 4,076,698. Density of the polyethylene useful in the present invention is as defined in ASTM D-1248.
Many uses have been disclosed for grafted olefin polymers and copolymers. These include use of a maleic anhydride grafted olefin polymer as a blend component in the sheath of a bicomponent fiber as disclosed in U.S. Pat. No. 4,950,541.
U.S. Pat. No. 4,684,576 discloses a blend of maleic anhydride grafted HDPE with ungrafted LLDPE with the blend being in the form of a layer adhered to at least one substrate.
U.S. Pat. No. 4,452,942 discloses blends of X-methyl bicyclo(2.2.1)hept-5-ene-2,3-dicarboxylic acid anhydride (XMNA)-grafted LLDPE and HDPE with ungrafted olefins (e.g., LLDPE) that have such high adhesion to tin-free steel that the layers could not be separated. The comonomer in the LLDPE is not identified.
While these references disclose adhesion of the grafted olefin polymers at room temperature and/or at elevated temperatures, these references fail to teach or describe specifically how to consistently maintain or improve adhesion of the grafted olefin polymer at elevated temperatures. Elevated temperatures may be encountered for example in multilayer structures useful for storing and subsequently cooking foods without changing containers.